Mass spectrometry is an important technique in the field of chemical analysis. In particular, mass spectrometry may be used to analyse and identify organic compounds. The analysis of organic compounds using mass spectrometry is challenging as organic compounds can range in mass from tens of amu up to several hundred thousand amu.
In general, a mass spectrometer comprises an ion source for generating ions, various lenses, mass filters, ion traps/storage devices, and/or fragmentation device(s), and one or more mass analysers. Mass analysers may utilise a number of different techniques for separating ions of different masses for analysis. For example, ions may be separated temporally by a Time of Flight (ToF) mass analyser, spatially by a magnetic sector mass analyser, or in frequency space by a Fourier transform mass analyser such as an orbital trapping mass analyser.
For orbital trapping mass analysers and ToF mass analysers, ions to be analysed may be grouped as ion packets prior to injection into the mass analyser. An extraction trap may be provided in order to form an ion cloud (ion packet) of analyte ions to be analysed with a suitable space and energy distribution for injection into an orbital trapping or ToF mass analyser. Examples of injecting ions into mass analysers using extraction traps are disclosed in U.S. Pat. Nos. 7,425,699 and 9,312,114.
Known extraction traps utilise a combination of potential and pseudopotential wells in order to confine analyte ions within the extraction trap. When confining analyte ions in an extraction trap, Coulombic repulsion, or space charge, between the trapped analyte ions opposes the confining forces of the applied potential and pseudopotential wells. As the number of trapped analyte ions increases, the potential resulting from the space charge increases. This space charge potential opposes the confining potential of the extraction trap. As the space charge potential approaches that of the potential well depth, the spatial distribution of the analyte ions in the ion trap increases rapidly. Large spatial distributions of analyte ions are undesirable, as this may negatively affect the transmission and/or resolution of the mass analyser.